1. Field of the Invention
The present invention relates to nickel powders and a method for preparing the same.
2. Description of the Related Art
Nickel is a transition metal that belongs to the iron group in Period 4, Group VIII of the periodic table and is a crystalline substance with high melting point and excellent malleability.
Nickel powders are a particle-phase metallic nickel material. Nickel powders can be used as, for example, a material for inner electrodes in electronic devices such as multilayer ceramic capacitors (MLCCs), a magnetic material, an electrical contact material, a conductive adhesive material, or a catalyst.
Nickel is known as a representative of ferromagnetic substances. Ferromagnetic substances are those that are strongly magnetized in the direction of a magnetic field applied, and retain magnetization even when the magnetic field is removed.
When a non-magnetized ferromagnetic substance is exposed to an increasing magnetic field, magnetization occurs slowly at an early stage, which is called initial magnetization. Thereafter, the rate of magnetization increases and saturation occurs. When a magnetic field is decreased at a saturation state, magnetization is reduced. However, the reduction course of magnetization is different from the increase course of magnetization. Also, even when a magnetic field becomes zero, magnetization does not reach zero, which is called residual magnetization. When the direction of a magnetic field is reversed and the intensity of the reverse magnetic field is increased, magnetization reaches zero and then the direction of the magnetization is reversed. Thereafter, the reverse magnetization gradually reaches a saturation state. At this time, even when a magnetic field becomes zero, magnetization does not reach zero and reverse residual magnetization remains, thereby creating a closed curve which does not pass through the origin. The closed curve is called a magnetization curve. The magnetization curve is closely related with a magnetic domain structure.
It is known that a ferromagnetic substance has an increased magnetic moment, which is a causative factor of magnetization, produced by parallel electron spins. Also, it is assumed that a ferromagnetic substance has magnetic domains which are clusters of parallel spins. When a magnetic field is applied, magnetic domains are aligned in the direction of the magnetic field. Even when a magnetic field is removed, the orientations of the magnetic domains are maintained for a long time, thereby generating residual magnetization. In this regard, when a temperature of a ferromagnetic substance is raised, the alignment of electron spins in the ferromagnetic substance is randomized by thermal motion. As a result, the ferromagnetic substance loses ferromagnetism and is transformed into a paramagnetic substance. The temperature is called the Curie temperature. The magnitude of a reverse magnetic field necessary to reduce the magnetization of a magnetized magnetic substance to zero is the coercive force.
Magnetic properties of bulk nickel are as follows: about 353° C. of the Curie temperature, about 0.617 T of saturation magnetization, about 0.300 T of residual magnetization, and about 239 A/m of coercive force.
Allotropes of nickel that have been known until now include metallic nickel with a face-centered cubic (FCC) crystal structure and metallic nickel with a hexagonal close packed (HCP) crystal structure.
Almost all common nickel powders are ferromagnetic substances with a FCC crystal structure. There are very rare reports of preparation of nickel powders with a HCP crystal structure. It has been predicted that the nickel powders with a HCP crystal structure are also ferromagnetic substances.
Based on the Stoner theory, D. A. Papaconstantopoulos et al. predicted that HCP nickel must be a ferromagnetic substance [D. A. Papaconstantopoulos, J. L. Fry, N. E. Brener, “Ferromagnetism in hexagonal close packed elements”, Physical Review B, Vol. 39, No. 4,1989. 2. 1, pp 2526-2528].
With respect to preparation of inner electrodes for electronic devices that are representative application areas of nickel powders, conventional ferromagnetic nickel powders have the following disadvantages.
First, when nickel powders contained in pastes for nickel inner electrode formation by a printing method exhibit magnetism, the nickel powders are attracted to each other like magnets and agglomerated, which renders uniform paste formation difficult.
Second, an ultra-high frequency band is used in electronic devices with development of the mobile communication and computer technologies. However, magnetic substances have a high impedance value at such a high frequency band.
These problems can be solved by using non-magnetic nickel powders.